weclome to the board.
how long has the tank been set up? what is in the tank? fish? inverts? the hermits will be part of your clean up crew, so how many you need may depend on what else you have to clean up.
I'd say start with 5-10 then get more if you need more. If we found some for a good price somewhere we'd pick up a few. I think we only have about 4 in out 55gl right now. (we haven't added any in about 2 years).
hth